Add the following line as a source in your graphical package manager (In Ubuntu: System->Administration->Synaptic package manager; then Settings->Repositories and then finally Third party software), or edit the file /etc/apt/sources.list and add it there.

deb http://apt.etherpad.org all .

apt-get update
apt-get install etherpad

Answer the questions (if any, depends on your system settings)

/etc/init.d/etherpad start

Other Linux distributions
Download the .tgz and unpack it
Follow instructions in README